A rotary head assembly of a VTR is generally composed of an upper rotary cylinder 2 and a lower fixed cylinder 3 as shown in FIG. 1, and a magnetic tape runs at a constant speed being helically wound around the rotary head assembly of about 180.degree.. Numeral 4 refers to a step part (called lead hereinafter) which guides the running position of the tape in its widthwise direction. A video head 5 is fixed to the upper rotary cylinder 2 and rotates at high speed (1800 rpm in the VHS system VTR) to record and reproduce picture signals on the magnetic tape. Numerals 6, 7, 8, 9 are tape guide posts.
The behavior of magnetic tape on the rotary head assembly having such constitution appears to be simple, but is actually extremely complicated. FIG. 2 is a schematic model representing a sectional view of a conventional rotary head assembly and the behavior of magentic tape 1 on it. Since the upper rotary cylinder 2 rotates at high speed, an air film is formed between the tape 1 and the outer circumference of the upper rotary cylinder 3 due to inclusion of air, so that the tape 1 receives a buoyancy in the upper rotary cylinder area. In the lower fixed cylinder area, on the other hand, since the relative speed between the magnetic tape 1 and the outer circumference of the lower fixed cylinder 3 is extremely small (11.0 mm/sec in 6 o'clock mode of VHS system VTR), such air film is not formed, and the magnetic tape 1 slides while contacting with the outer circumference of the lower fixed cylinder. Meanwhile, the magnetic head 5, in order to exchange signals with the magnetic tape 1, slides at high speed on the magnetic tape at an adequate contact pressure. Therefore, on the rotary head assembly, the magnetic tape is subject to a buoyancy in the upper rotary cylinder area and runs almost without contact, whereas, in the lower fixed cylinder area, the tape runs while receiving a sliding frictional resistance. That is, the dynamic situations vary significantly depending on the position of the tape in its widthwise direction. Further, since the magnetic tape is wound around the rotary head assembly helically, its dynamic situations vary also in the longitudinal direction of the tape. In particular, at the contact terminal end between the magnetic tape and rotary head assembly, the contact surface area with the lower fixed cylinder is increased, and the tape buoyancy is inferior. Yet, the upper rotary cylinder and lower fixed cylinder are assembled almost coaxially, but the dynamic state of the tape differs significantly depending on the coaxiality and the difference in diameter of the two cylinders. In FIG. 2, numeral 11 is a shaft, on which the upper rotary cylinder 2 is fixed by way of a disc 10. Numerals 12, 13 are ball bearings which rotate and support said shaft, 15 is a spacer, and 14 is a collar to fix the ball bearings by preloading.
Thus, the magnetic tape runs on the rotary head assembly under complicated dynamic state, but involves the following problems. That is:
(1) Tape chirping due to sliding at lower fixed cylinder--this is a kind of frictional vibration accompanying the sliding between tape and cylinder, and vibrations of about hundreds to several kilohertz occur in a running tape. This phenomenon is likely to occur when the tape running speed is low and the ambient humidity is high, which gives rise to color unevenness of picture and jitters.
(2) Sticking of tape to upper and lower cylinders--generally, coarseness of tape surface is in the order of hundreds of .ANG. and the surface is very smooth, and the cylinder surface is also finished smoothly (to surface coarseness of 0.1 to 1 S) in order to avoid occurrence of tape damage. When two surfaces of extreme smoothness contact with each other, they tend to suck each other. In the tape running system of VTR, such tendency appears as the phenomenon of tape sticking on the rotary head assembly, which may finally disable the tape to run. This phenomenon is likely to occur in damp atmosphere.
(3) Tape damage and wear due to contact between parts of the rotary head assembly and magnetic tape--tape damage not only causes dropout of magnetic recording signals, but also accelerates the wear of sliding parts with the magnetic dust falling off, or the magnetic dust may deposit on the head gap of the video head to clog it, thereby disabling to record and reproduce.
All these problems are caused by contact and sliding of the magnetic tape on the rotary head assembly, in particular, the lower fixed cylinder.
On the other hand, in the recent downsizing trend of VTR, in order to increase the signal recording density of magnetic tape, the conventional tapes prepared by coating with magnetic powder of iron oxide or chroium dioxide together with resin binder are being gradually replaced by alloy tapes or vacuum deposition tapes as magnetic tapes. While the magnetic surface of conventional tapes is composed of resin and magnetic powder, that of these new tapes is literally a metallic surface, and the coefficient of friction due to sliding with the stainless steel increased from the conventional 0.2 to 0.5 approximately. Therefore, the above problems accompanying the sliding at the rotary head assembly where the magnetic surface slides and contacts become more serious than in the conventional tapes.
As a means to present a tape guide allowing the tape to run smoothly, the cylinder constituted as represented in FIGS. 3 A, B, C was proposed in the Japanese unexamined patent publication Sho. No. 52-24507. That is, in FIGS. 3 A, B, C, numeral 16 is an air guide hole, 17 is an air chamber formed annularly on the lower face of upper rotary cylinder 2, and 18 is multiple grooves with the either ends communicating with the air chamber 17 and the other ends reaching the outer circumference of the upper rotary cylinder 2. Therefore, when the upper rotary cylinder 2 rotates in the arrow direction, the air flowing in from the air guide hole 16 is accelerated by the centrifucal force of rotation through the air chamber 17 and is blown out from the outer circumference of the lower end face of the upper rotary cylinder 2, exerting a power to lift the tape above from the outer circumference of the tape guide thereby allowing the tape to run more smoothly. However, in the conventional example as shown in FIG. 3, nothing is mentioned about the pressure occurring due to relative movements of the lower end face of the upper rotary cylinder 2 forming the grooves 18 and the upper end face of the lower fixed cylinder on its opposing plane, that is, the effect of generation of pressure due to shearing force derived from the viscosity of air, and the grooves 18 act only as the blades of a centrifugal blower, and are only meant to lift the tape by the inertial force of the air accelerated by the centrifugal force. In this method, therefore, it is insufficient to form a desired air film between the tape and cylinder, overcoming the tape tension, and it is difficult, in particular, to avoid contact of the tape at the winding end part of the tape around the cylinder, that is, at the lower fixed cylinder near the exit of the tape.
In another conventional example, as disclosed in the Japanese unexamined patent publication Sho. No. 52-24506, a cylinder constitution was proposed to increase the pressure at the slits and promote the floating of the tape by forming slits of specified width between the upper and lower cylinders and installing pump-in grooves in the slits, but, in this case, the creation of pressure by slits occurs only in the slit area and the pressure rises continuously from the slit inlet to the groove closing end part, while the pressure between the slit inlet and the magnetic tape or cylinder is equal to the atmospheric pressure. Therefore, there is no effect to promote the floating of the magnetic tape.